High Speed Downlink Packet Access, HSDPA, is a technique for accessing downlink data packets in a high speed. The HSDPA system has the following characteristics. 2 ms short frame is adopted, a physical layer Hybrid Automatic Repeat Request, HARQ, technique and an Adaptive Modulation and Coding, AMC, technique are adopted in the physical layer, 16-Duadrature Amplitude Modulation, 16QAM, is introduced to perform high-order modulation to improve spectrum utilization, and code division and time division are adapted to realize shared channel scheduling on each User Equipment, UE.
The HARQ technique adopts Stop-And-Wait, SAW, protocol, in which after data is transmitted by a base station to the UE, the UE is required to feed back Acknowledgement/Negative Acknowledgement, ACK/NACK, and the base station can acquire whether the UE has correctly received the data according to the ACK/NACK, so as to determine whether to retransmit the data to the UE or to transmit new data to the UE.
In HSDPA, two physical channels, namely, High Speed Shared Control Channel, HS-SCCH, and High Speed Physical Downlink Shared Channel, HS-PDSCH, are added to the downlink. The HS-SCCH is adapted to bear the required signaling for demodulating the associated data channel HS-PDSCH, and the HS-PDSCH is adapted to bear data information of the user.
In HSDPA, a physical channel, namely, a High Speed-Dedicated Physical Control Channel, HS-DPCCH, is added to the uplink. The channel is adapted to bear the information fed back by the user about whether the downlink data frame HS-PDSCH is correctly received, i.e. the ACK/NACK, or is adapted to bear a Channel Quality Indication, CQI.
Meanwhile, in the HSDPA system, a Medium Access Control-high speed, MAC-hs, entity is added to a Medium Access Control, MAC, layer for supporting the flow control of the HSDPA, performing quick scheduling/priority management, HARQ, and transport format and resource indicator, TFRI, selection.
In Wideband Code Division Multiple Access, WCDMA, network structure, the MAC layer, a Radio Link Control, RLC, layer, and other upper layers are located above the physical layer, the physical layer and a part of the MAC entity of the HSDPA are located in the base station, the RLC layer and the entities over the RLC layer are located in a Radio Network Control, RNC, and the base station and the RNC has a standard interface data transmission manner there-between.
Before the physical layer HARQ retransmission is introduced into the WCDMA system, the system ensures the correct transmission of the service data by the ARQ retransmission of the RLC layer and the retransmission of the upper layers. After the HARQ physical layer retransmission is introduced into the WCDMA system, when the physical layer retransmission times reaches maximum retransmission times given by the system, but the physical layer still does not correctly receive the data, the RLC layer of the receiving end requests an Automatic Repeat Request, ARQ, retransmission according to a Serial Number, SN, state of a received Protocol Data Unit, PDU. Once receiving the request, the transmitting end starts the ARQ retransmission.
The detailed retransmission process includes the following steps:
1. The RLC layer of the transmitting end divides or combines the data unit transmitted by the upper layer to generate the appropriate RLC Acknowledgement Mode Data, AMD, PDU, and numbers and transmits the RLC AMD PDU to the physical layer of the transmitting end, and buffers the RLC AMD PDU in the retransmission buffer area.
2. After processing the received data unit, the physical layer of the transmitting end transmits the data unit to the physical layer of the receiving end, and the transmitting end waits for the HARQ feedback information transmitted by the receiving end.
3. The receiving end feeds back whether each data unit is correct to the transmitting end. If the data unit is correct, the receiving end feeds back the HARQ ACK to the transmitting end, and delivers the data to the RLC of the receiving end, and if the data unit is incorrect, the receiving end feeds back HARQ NACK to the transmitting end.
4. The transmitting end determines whether to perform the HARQ retransmission according to the HARQ feedback information, the maximum retransmission times, and other information. If the HARQ maximum retransmission times is not exceeded, the retransmission continues, otherwise, the transmitting end terminates the HARQ retransmission and waits for an Automatic Repeat Qequest, ARQ, retransmission of the RLC.
5. The RLC layer of the receiving end determines whether the RLC AMD PDU requires the ARQ retransmission according to the received data, the timer, and other information, and feeds back an ARQ STATUS REPORT to the RLC of the transmitting end.
6. The RLC of the transmitting end determines whether to initiate the ARQ retransmission according to the received STATUS REPORT and the local information, and the retransmission step is the same as the above.
In the above RLC ARQ retransmission solution, in order to determine the transmission time and the content of the STATUS REPORT, a great number of timers and counters are configured in the RLC. The message format of the STATUS REPORT includes BITMAP, LIST, and RLIST, and the triggering mechanism includes a timing report and an error report, and the operation process is quite complex.
In addition, the process of feeding back the BITMAP information to the transmitting end by the receiving end is finished on the RLC layer, particularly for the ARQ retransmission. If the RLC of the receiving end determines that data loss occurs, for example according to the SN, the RLC of the receiving end transmits the STATUS REPORT for requesting the data retransmission. Then, at the transmitting end, the RLC must detect the STATUS REPORT transmitted by the receiving end on the RLC layer, so as to determine whether an RLC AMD PDU requires the retransmission, and therefore, the process takes much time.
In order to eliminate the disadvantages of the above technical solution, the prior art provides a new two-layer retransmission mechanism. Under the mechanism, the problems which may not be solved by the HARQ retransmission still must be solved by the ARQ retransmission, but the detailed retransmission process is changed. The detailed process is as shown in FIG. 1, which includes the following steps:
1. 101a, 101b and 101c: The RLC of the transmitting end divides or combines the data transmitted by the upper layer to generate the appropriate data unit PDU, then delivers the data unit PDU to the lower layer for transmission, and buffers the data unit PDU in the retransmission buffer area.
2. 102a, 102b and 102c: After processing the data unit, the lower layer of the transmitting end transmits the data unit to the receiving end and waits for HARQ acknowledgement of the receiving end.
3. The receiving end performs the HARQ feedback on each received data unit. If the data unit is correct, the receiving end feeds back the HARQ ACK to the lower layer of the transmitting end 103c. and transmits the data to the RLC of the receiving end. 103a: If the data unit is incorrect, the receiving end feeds back the HARQ NACK to the lower layer of the transmitting end. 104a: After receiving the HARQ NACK, the lower layer of the transmitting end determines whether the HARQ maximum retransmission times are reached. If the HARQ maximum retransmission times are not reached, the process returns to Step 2 to transmit the PDU to the receiving end again. 105a: If the HARQ maximum retransmission times is reached, the lower layer of the transmitting reports the local NACK to the RLC of the transmitting end.
4. After the RLC of the transmitting end receives the local NACK, the transmitting end starts the ARQ retransmission process of the data unit.
103b: In the above process, if the lower layer of the receiving end finds the HARQ NACK/Discontinuous Transmission, DTX->ACK error, that is, the HARQ NACK signaling fed back by the receiving end to the transmitting end is changed to the HARQ ACK signaling during the transmission process, or the transmitting end detects the HARQ ACK signaling by mistake under a situation that the receiving end does not feed back the HARQ. 104b: The receiving end transmits an error indication information of HARQ NACK/DTX->ACK to the lower layer of the transmitting end. 105b: After receiving the indication, the lower layer of the transmitting end reports the local NACK to the RLC of the transmitting end (upper layer).
After receiving HARQ ACK of the receiving end, the lower layer of the transmitting end determines whether to transmit the local ACK to the RLC of the transmitting end according to the timer. 104c: If the error indication information of NACK/DTX->ACK transmitted by the receiving end is not received when the preset time duration is exceeded, it is considered that the receiving end correctly receives the data in deed. 105c: The lower layer of the transmitting end transmits the local ACK to the RLC of the transmitting end.
In the above retransmission solution, the ARQ feedback signaling of the upper layer is assisted by the lower layer, which lowers the complexity of operation of the upper layer, meanwhile, the assistant signaling transmitted by the lower layer also reduces the retransmission feedback time. However, the technical solution has a problem of missing the HARQ NACK/DTX->ACK error. The reason is that the lower layer of the transmitting end must determine whether the HARQ maximum retransmission times is reached according to the number of the received HARQ NACK, so as to determine whether to report the ARQ local NACK to the upper layer. Further, the receiving end determines whether the HARQNACK/DTX->ACK error occurs, the determination is made under the situation that the new data is received before the HARQ maximum retransmission times is reached. Therefore, if the HARQ NACK/DTX->ACK occurs during the final HARQ retransmission process, the lower layer of the receiving end may not find the HARQ NACK/DTX->ACK error, and may not transmit the error indication information of HARQ NACK/DTX->ACK to the transmitting end, such that the lower layer of the transmitting end considers that the data is correctly transmitted, the transmitting end transmits the local ACK to the upper layer of the transmitting end, the upper layer of the transmitting end deletes the data from the retransmission buffer area. Therefore, the data may not be retransmitted, and the receiving end may not receive the data through the ARQ retransmission, thus resulting in an error which cannot be corrected.